The wildcard pairing became the first team since Jana Novotna and Jim Pugh in 1989 to retain the crown and the first Australian pair to do so since Margaret Court and Ken Fletcher 62 years ago.
"I can't believe we're in this position right now," said Gadecki after claiming her second Grand Slam title alongside Peers, who secured his fourth.
"To hold the trophy again this year is incredible. I knew we could do it, but I didn't think we could really do it."
Mladenovic and Guinard broke Peers in the opening game and consolidated the advantage, but the Australians fought back to level at 2-2, setting the tone for a roller-coaster opening set featuring five breaks of serve.
Mladenovic was broken while serving for the set but the French pair managed to close it out 6-4 on Guinard's serve.
Spurred on by chants of "Aussie, Aussie, Aussie," the Australian duo settled down in the second set and broke early to go 3-1 up.
They managed to hold serve and Peers sealed the second set with an ace to force a 10-point tiebreak as the Rod Laver Arena crowd erupted.
Mladenovic's early double fault - which struck her partner's back on the second serve - set the tone for a topsy-turvy finale with the serving team losing five consecutive points before the French duo edged ahead 7-5.
The Australians clawed their way back to take the lead, however, with Peers delivering a forehand winner to earn two championship points.
The title was theirs when Guinard, playing in his first Grand Slam final, found the net.
"Winning the Grand Slam once is super difficult, but defending the title is a very, very big task. So big congratulations," said Mladenovic, a nine-times Grand Slam winner in doubles and mixed doubles.
"A couple of months ago I was barely walking, I was on crutches. The one who believed in me was him (Guinard), he was already planning and playing with me. So thank you very much for believing in me."
